How close were Ardi and plum?
They were very close, up until Ardi was summoned to the palace to become their resident Enchantress.
Ardi and Plum were actually born Soojin and Hyejin Joo, in a far-off country, and were forced to flee to Vesuvia to escape persecution for their magical practice. They changed their names to Gardenia and Plumeria Pearlforger (Jinjoo means “pearl”), and opened up another magic practice.
They ran it together for a number of years, and Plum took Asra on as an apprentice, before Gardenia received a summons from the palace to perform a consultation. After performing excellently, she was invited to stay as a resident adviser, and was unable to stay in as much contact with her sister as before.
Ironically, the sisters did briefly reunite while they both were quarantined on Lazaret, although they were both so mentally incapacitated that they barely recognized each other.